1. Field of the Invention
The present invention relates to an EL (electroluminescence) element and an EL phosphor used therefor, a thin film multilayer structure of an oxide EL phosphor in particular.
2. Related Background Art
In recent years, studies of thin film EL elements as large-size, lightweight flat display panels have been actively under way. FIG. 6 is a perspective view schematically showing an example of conventional EL display. This EL element 100 is a monochromatic EL display comprising a phosphor thin film made of yellowish-orange-emitting, manganese-doped zinc sulfide (ZnS:Mn) disposed between two thin film insulating layers. It has a so-called double insulating structure and has already been in actual use.
Specifically, a lower electrode 13a having a predetermined pattern is formed on a substrate 12 made of glass, whereas a dielectric thin film is formed as a lower insulating layer 14a on the lower electrode 13a. A light-emitting layer 15 made of a phosphor thin film, and an upper insulating layer 14b are successively formed on the lower insulating layer 14a. On the upper insulating layer 14b, an upper electrode 13b is formed with a predetermined pattern so as to constitute a matrix electrode with the lower electrode 13a. For making the light-emitting layer 15, the phosphor thin film is usually formed and then is annealed at a temperature not higher than the strain point of the substrate 12, in order to improve luminance.
Also, a structure using ceramics for the substrate 12 and a thick film dielectric layer for the lower insulating layer 14a has recently been proposed. Further, a device structure using a BaTiO3 thin sheet, formed with an electrode on one side, exhibiting a high dielectric constant as the lower insulating layer 14a and substrate 12 has been proposed. Since such a structure uses ceramics such as alumina and BaTiO3 as the substrate, the light-emitting layer 13 made of a phosphor thin film can be annealed at a higher temperature, thus achieving higher luminance.
Also, since a dielectric layer constituted by a thick film or thin sheet is used as the lower insulating layer 14a, the above-mentioned structure is stronger against dielectric breakdown than those using a thin film for the lower insulating layer 14a, whereby a highly reliable device can be obtained.
Here, the double insulating structure is not always necessary, whereas the structure may have a thick film or thin sheet dielectric layer alone as its insulating layer.
Meanwhile, displays currently in use for PCs, TVs, and other display purposes are required to show colors. Though thin-film EL displays using sulfide phosphor thin films are excellent in reliability and environmental resistance, they have not yet been considered suitable for use in color displays, since characteristics of EL phosphors emitting three primary colors of red, green, and blue are not sufficient.
Here, the following materials have been listed as candidates for materials for red, green, and blue, and their studies have been under way. Examples of blue-emitting phosphors include SrS:Ce using SrS as a matrix material and Ce as a luminescent center, SrGa2S4:Ce, and ZnS:Tm. Examples of red-emitting phosphors include ZnS:Sm and CaS:Eu. Examples of green-emitting phosphors include ZnS:Tb and CaS:Ce.
These phosphor thin films emitting three primary colors of red, green, and blue have not yet been considered sufficient in terms of light-emitting luminance, efficiency, and color purity, so that no color EL elements using them have come into actual use yet. In particular, though green emission with a relatively high luminance has been attained by using SrS:Ce, its color purity is too shifted to the green side to be used for blue in a full-color display, thus demanding a blue-light-emitting layer with a higher blue color purity to be developed.
For solving this problem, as set forth in Japanese Patent Application Laid-Open Nos. HEI 7-122364 and HEI 8-134440, IEICE Technical Report, EID 98-113, pp. 19-24, and Jpn. J. Appl. Phys. Vol. 38 (1999), pp. L1291-1292, blue-emitting phosphors of thiogallate or thioaluminate type excellent in luminance and color purity have been under development.
Thus, sulfide composition systems expanded from zinc sulfide (ZnS) to two- or three-member sulfides have widely been used as phosphors for EL elements so far. However, sulfide materials in general have the drawback of being susceptible to water and moisture. For example, Al2S4 reacts with water in the air, thereby changing into Al2O3 while generating H2S. Therefore, when a phosphor made of a sulfide material is employed in an EL element, means for blocking water and protecting the element must be provided from the viewpoint of phosphor life. This will complicate the element structure.
Under these circumstances, the research and development of high-luminance EL elements using chemically stable oxide type phosphors in their light-emitting layers has recently been gaining momentum. For example, EL materials with various emission colors have been proposed in Display and Imaging, Vol. 8 suppl., pp. 83-93.